Polymers containing highly hydrophilic hydrogen bonding groups in molecules, which polymers are represented by poly(meth)acrylic acid and polyvinyl alcohol, have been publicly known as gas barrier polymers. With regard to films made up solely of such polymers, however, their gas barrier properties against oxygen and the like is deteriorated due to their hydrophilicity under high humidity conditions, although the gas barrier properties against oxygen and the like is excellent under a dry condition. Furthermore, films made up solely of such polymers have problems with resistance against humidity and hot water, and the problems include dissolution of them in hot water. The problems impose restrictions on industrial use of these polymers as gas barrier resin.
In order to solve the problems, Patent Literature 1 (claim 1 of Japanese Patent Laid-open Official Gazette No. Hei. 10-237180) has proposed a gas barrier resin composition which is characterized by including, in its chemical structure, ester bonds formed by reaction between poly(meth)acrylic acid and polyalcohol as well as ionic bonds formed by reaction between poly(meth)acrylic acid and multivalent metal ions. Patent Literature 1 has disclosed that limitation of the ratio between the ester bond and the ionic bond in a specific range makes it possible to obtain a gas barrier film with resistance against high-temperature water vapor and hot water. Furthermore, Patent Literature 1 has disclosed a method of forming ester bonds between poly(meth)acrylic acid and polyalcohol by means of heat treatment, and a method of forming ionic bonds between poly(meth)acrylic acid and multivalent metal ions by means of further dipping a mixture of poly(meth)acrylic acid and polyalcohol, which has been treated with heat, into water containing a multivalent metal compound.
Moreover, Patent Literature 2 (claim 1 of Japanese Patent Application No. Hei. 2002-121246) has disclosed a film whose raw materials are a polycarboxylate-based polymer (A) and a multivalent metal compound (B), and which has a peak ratio (A1560/A1700) of infrared absorption spectrum not lower than 0.25.
According to Patent Literature 1, however, in order to fully develop oxygen-gas barrier properties and resistance against high-temperature water vapor and hot water, a mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer needs to be denatured by means of process operations including heat treatment. If the mixture is not fully denatured, this makes it difficult to obtain resistance against high-temperature water vapor and hot water, or moisture proofness, although existence of the polyalcohol in the mixture enables excellent gas barrier properties to be obtained. With regard to the film disclosed by Patent Literature 2, improvement in the moisture proofness is further desired, although excellent gas barrier properties are obtained. Heretofore, metals, metallic foils, glass and the like have been used as moisture-proof materials and highly moisture-proof materials in particular. However, application of plastic materials has been anticipated for electronic parts, members of electronic equipment and the like, and packaging materials for them, with transparency and flexibility of the plastic materials taken into consideration. Particularly, development of plastic materials with moisture proofness and a gas barrier against oxygen has been awaited.
An object of the present invention is to provide a film, which includes at least a multivalent metal salt of a polycarboxylate-based polymer (A), and which has moisture proofness along with oxygen-gas barrier properties, and is to provide a method of manufacturing